Not a bad idea but i believe to get that good and deep demerara flavour you need to add also a dark demerara rum as well and i would suggest - 2:2:1 of Goslings 151 & El Dorado High Strength and El Dorado 15.

I've tasted 3 overproof rums: Bacardi of course, Lemon Hart 151, and the El Dorado High Strength. Of the 3, the Lemon Hart is almost sippable, with a strong molasses component to its taste. The burn is there, but you don't notice it as much. The El Dorado is very vanilla tasting, and very hot. Definitely a mixer. I couldn't see using it in place of the Lemon Hart in any situation, the flavor of each is so distinct.